What is a molar extinction coefficient with regard to pulse oximetry?

A molar extinction coefficient is a measure of the absorption of light at a particular wavelength by a substance of known concentration. Specifically, it is the measure of how much light is absorbed per unit length (i.e. centimeters) by one mole of the substance.

In pulse oximetry, the molar extinction coefficient is used to calculate the concentration of oxygenated and deoxygenated hemoglobin in blood based on the absorption of light at two different wavelengths (usually 660 nm and 940 nm) by the hemoglobin molecules.

Figure: Jubran, Critical Care, 2015

Keywords: molar extinction coefficient, extinction coefficient, hemoglobin, 

References: Jubran, Critical Care, 2015 Chan et al, Respiratory Medicine 2013